<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>全屏——退出全屏</title>
    <style>
        body {
            background-color: black;
        }

        .fullScreen {
            position: absolute;
            top: 20px;
            right: 30px;
        }

        .exitFullScreen {
            display: none;
            position: absolute;
            top: 20px;
            right: 30px;
        }

        .main {
            color: orangered;
            font-size: 60px;
            margin: 150px auto;
            text-align: center;
        }
    </style>
</head>
<body>
<button class="fullScreen">全屏</button>
<button class="exitFullScreen">退出全屏</button>
<div class="main">我现在非全屏状态</div>

<script src="./jquery-2.1.1.min.js"></script>
<script src="./script.js"></script>
</body>
</html>